A high frequency transformer for a phase-shifted full-bridge converter must balance soft-switching energy against duty-cycle loss, circulating current, rectifier stress, and thermal performance. Leakage inductance can help charge and discharge switch capacitances for zero-voltage switching, but excessive leakage reduces effective secondary duty cycle and increases commutation stress.
Define the complete operating range
Provide minimum and maximum input voltage, output voltage, power range, switching frequency, phase-shift limits, controller dead time, rectifier type, expected ZVS range, and cooling. Light load, startup, current limit, and burst or skip modes can produce magnetic conditions that are different from rated operation.
Control leakage inductance deliberately
Some PSFB designs use transformer leakage as part of the commutation inductance; others add an external series inductor for tighter control. Integrated leakage depends on winding placement, interleaving, margins, and production tolerance. If it is a functional design element, the drawing must specify the measurement connection, frequency, and acceptable range.
More interleaving generally reduces leakage, but can increase primary-secondary capacitance and common-mode current. The useful construction balances switching transition energy with EMI and isolation requirements.
Account for duty-cycle loss
During secondary current commutation, leakage inductance delays the transfer of voltage to the output. This reduces effective duty cycle, especially at high current. The lost interval changes with load, input voltage, leakage, and rectifier behavior. Turns ratio selection must include that loss rather than using the ideal bridge waveform alone.
Prevent flux walking
Small asymmetries in gate timing, device drops, drive, or current can create unequal positive and negative volt-seconds. Repeated imbalance can walk the core toward saturation. Verify primary voltage and current symmetry during steady state, transients, and current limiting. Adequate flux margin and controller protection remain necessary even with nominally bipolar excitation.
Evaluate copper loss and circulating current
PSFB converters can carry primary circulating current during intervals that transfer little output power. RMS winding current may therefore be higher than an output-power calculation suggests. Secondary rectifier commutation and proximity effects can further increase AC winding loss. Foil, litz, and sectional winding choices should follow measured current waveforms and frequency.
Prototype verification checklist
- Magnetizing and leakage inductance with documented fixtures
- Primary-current symmetry and core flux margin
- ZVS behavior across input voltage and load
- Effective duty-cycle loss at rated current
- Primary switch and secondary rectifier voltage stress
- Winding, core, and terminal temperature rise
- Common-mode current and EMI with final grounding
When requesting a high frequency transformer for a PSFB converter, share current waveforms, commutation strategy, intended leakage contribution, controller timing, and rectifier topology. BaoHui Tech can then align winding construction and tolerances with the actual soft-switching requirements.
Frequently asked questions
Should transformer leakage provide all ZVS energy?
Not always. An external inductor can offer more independent control, while integrated leakage saves components but increases winding-tolerance sensitivity.
Why does effective duty cycle shrink at high load?
More secondary current requires more time to commutate through the leakage inductance, delaying full secondary voltage.
Can a full-bridge transformer saturate from flux imbalance?
Yes. Timing and voltage asymmetry can create net volt-seconds over repeated cycles, so current symmetry and protection must be checked.